1. Field of the Invention
The present invention generally relates to an emergency power system, and to a system for automatically detecting whether not a failure of a single cell occurs in a battery for use in the system.
2. Description of the Related Art
FIG. 10 illustrates a float charging system employing a lead acid battery 30 in an emergency power system. Ordinarily, DC power is supplied from a rectifier 31 to a DC load 32. A constant voltage is applied to the lead acid battery 30. Thus, a self-discharged quantity of energy is charged into the lead acid battery 30. Consequently, the lead acid battery 30 is continually kept in a fully charged condition. In the case of an emergency, for example, in the event of an outage of the rectifier 31, electric power is supplied from the DC load 32 to the lead acid storage battery 30. Thus, in the case of an emergency, for instance, in the event of a power failure, the conventional emergency power system can supply electric power to the DC load without an outage of power to the DC load.
However, the conventional emergency power system has drawbacks in that the system requires a large space for placing a lead storage battery therein, due to a low energy density thereof, and thus, the conventional system cannot be installed in a narrow place having limited space. Moreover, the conventional emergency power system has additional drawbacks in that the service life of the lead acid storage battery is short, which causes the frequency of replacement of the battery to be relatively high. Furthermore, the cost of the lead acid battery is relatively high.
The invention is accomplished in view of the aforementioned drawbacks. Accordingly, an object of the invention is to provide a space-saving emergency power system that can be placed in a narrow place having limited space. Further, another object of the invention is to provide a low-cost emergency power system employing a battery that is long in service life and low in frequency of replacement. Moreover, another object of the invention is to provide a system for detecting whether or not a single cell failure occurs in a battery used in an emergency power system.
To achieve the foregoing objects, according to an aspect of the invention, there is provided an emergency power system that includes a circuit having a DC load, a rectifier, and a battery, which are electrically connected to one another. The circuit is adapted so that normally, DC power is supplied from the rectifier to the DC load, and the battery is charged. In an emergency, such as a power failure, in which an outage of the rectifier occurs, electric power is automatically supplied from the battery to the DC load. In this system, the battery is a sodium sulfur battery.
Further, in an embodiment of the invention, preferably, a rectifier output voltage controller is provided between the rectifier and the sodium sulfur battery for lowering, when the voltage of the sodium sulfur battery reaches a level that is equal to or higher than the predetermined charging voltage, an output voltage of the rectifier thereby to automatically discharge energy from the sodium sulfur battery to the DC load. The rectifier output voltage controller also operates to raise, when the voltage of the sodium sulfur battery reaches a level that is equal to or lower than the predetermined discharging voltage, an output voltage of the rectifier to thereby charge the sodium sulfur battery.
Further, in an embodiment of the emergency power system of the invention, preferably, the predetermined charging voltage and the predetermined discharging voltage of the sodium sulfur battery are within a discharge depth range in the case that the composition of an active material accommodated in a positive electrode chamber of each of sodium sulfur single cells is in a two-phase region.
Further, in an embodiment of the invention, it is preferable that the sodium sulfur battery is an assembled battery obtained by constituting strings, each consisting of a predetermined number of series-connected single cells, and then parallel-connecting a predetermined number of the strings. An output voltage of the rectifier is controlled by monitoring whether or not a module voltage reaches the predetermined charging voltage and whether or not the module voltage reaches the predetermined discharging voltage.
Furthermore, preferably, an embodiment of the invention detects an occurrence of a malfunction of a single cell according to whether or not the difference xcex94V (=Vocvxe2x88x92Vm) between the no-load module open circuit voltage Vocv and the constant-resistance-load module voltage Vm is larger than an initially determined normal voltage difference xcex94Vn (xcex94V greater than xcex94Vn). An alarm is generated when a malfunction of a single cell occurs, thus maintaining the function of continually serving as an emergency power system.
The above-discussed detection of malfunctions is governed by the following principles. The voltage difference in the case of no occurrence of a malfunction is: xcex94V=I.R, where I is the module current, and R is the module resistance. When a malfunction of a single cell occurs, the resistance R rises, which, in turn, causes a corresponding increase in the voltage difference xcex94V.
According to the invention, there is provided another emergency power system, which includes a circuit consisting of a DC load, a rectifier and a battery electrically connected to one another and adapted so that under normal operating conditions, DC power is supplied from the rectifier to the DC load at a constant voltage, and the battery is charged. In an emergency situation, including a power failure, in which an outage of the rectifier occurs, power is supplied from the battery to the DC load. In this system, the battery is a sodium sulfur battery.
A controller having a timer is provided between the rectifier and the sodium sulfur battery and interrupts or connects an output of the rectifier. When the current from the sodium sulfur battery reaches a level that is equal to or lower than a predetermined charging current, an output of the rectifier is interrupted, and energy is automatically discharged from the sodium sulfur battery to the DC load for a predetermined period. After energy is discharged from the battery for the predetermined period, the output of the rectifier is connected to the sodium sulfur battery so as to charge the battery.
According to the invention, there is provided yet another emergency power system, which includes a circuit including a DC load, a rectifier and a battery electrically connected to one another and adapted so that under normal operating conditions, DC power is supplied from the rectifier to the DC load at a constant voltage, and the battery is charged. In an emergency situation, including a power failure, in which an outage of the rectifier occurs, power is supplied from the battery to the DC load. In this system, the battery is a sodium sulfur battery. The power system further comprises a controller which interrupts or connects an output of the rectifier. When a voltage of said sodium sulfur battery reaches a level that is equal to or higher than a predetermined charging voltage, an output of the rectifier is interrupted, and energy is automatically discharged from the sodium sulfur battery to the DC load. When the voltage of the sodium sulfur battery reaches a level that is equal to or lower than a predetermined discharging voltage, the output of the rectifier is connected to the sodium sulfur battery so as to charge the battery.
According to the invention, there is provided still another emergency power system, which includes a circuit consisting of a DC load, a rectifier and a battery electrically connected to one another. A diode, the forward direction of electrical conduction of which is a direction from the battery to said DC load, and a first switch are provided between the rectifier and the sodium sulfur battery. This emergency power is adapted so that under normal operating conditions, DC power is supplied from the rectifier to the DC load, and that in an emergency, including a power failure, in which an outage of the rectifier occurs, power is supplied from the sodium sulfur battery to the DC load through the diode.
This emergency power system further includes a control, provided between the sodium sulfur battery and the first switch, for putting, after discharge of the battery in an emergency, the first switch into a connected state and for charging the sodium sulfur battery to a predetermined discharge depth using power supplied from the rectifier. The first switch is thereafter interrupted to hold the sodium sulfur battery in a state in which power from the rectifier is prevented by the diode from being supplied to the battery.
Incidentally, in this emergency power system, practical means for putting the first switch into a connected state to thereby charge the sodium sulfur battery to a predetermined discharge depth by supplying power from the rectifier performs the charging until a charging current reaches a level that is equal to or lower than a set current, or until a module voltage is equal to or lower than a predetermined voltage. It is preferable for reducing a discharge starting voltage drop in an emergency that upon completion of charging, the discharge depth is increased by discharging a predetermined quantity of electricity (20 to 40 Ah).
An embodiment of the emergency power system further includes another circuit, provided between the circuit consisting of the diode and the first switch electrically parallel-connected and the sodium sulfur battery, for supplying discharging current of the sodium sulfur battery to a heater, and a second switch, provided between the heater and the sodium sulfur battery.
In this system, a temperature control operation is performed by controlling the on/off of the second switch according to the temperature of the sodium sulfur battery under the control of the controller, which is provided among the sodium sulfur battery, the first switch and the second switch. The sodium sulfur battery is maintained at a constant discharge depth and at a constant temperature by repeating one cycle consisting of steps of discharging energy from the sodium sulfur battery to the heater, and then charging energy from the rectifier to the sodium sulfur battery at each predetermined time interval under the control of the controller, which has a timer. The first switch is put into an interrupted state when the voltage or current of the sodium sulfur battery reaches a level that is equal to or higher than a predetermined voltage, or that is equal to or lower than a predetermined current, to thereby stop charging of the battery.
According to the invention, there is provided a detection system for automatically detecting an occurrence of a malfunction of a single cell in a battery used in an emergency power system. The detection system includes a first circuit consisting of a DC load, a rectifier, and a sodium sulfur battery electrically connected to one another. A second circuit is provided and consists of a diode, the forward direction of electrical conduction of which is a direction from the battery to the DC load, and a first switch parallel-connected to each other. A third circuit is provided between the second circuit and the sodium sulfur battery and supplies a discharging current from the sodium sulfur battery to a heater, and a second switch is provided between the heater and the sodium sulfur battery. The detection system is adapted so that under normal operating conditions, DC power is supplied from the rectifier to the DC load, and that in an emergency, including a power failure, in which an outage of the rectifier occurs, power is supplied from the sodium sulfur battery through the diode to the DC load.
This detection system further includes a controller provided among the sodium sulfur battery, the first switch, and the second switch. In this detection system, a temperature control operation is performed by controlling the on/off of the second switch according to a temperature of the sodium sulfur battery by the controller. The sodium sulfur battery is maintained at a constant discharge depth and at a constant temperature by repeating one cycle including the steps of discharging energy from the sodium sulfur battery to the heater, charging energy from the rectifier to the sodium sulfur battery at each predetermined time interval under the control of the controller, which has a timer, and putting the first switch into an interrupted state and stopping charging when a voltage or current of the sodium sulfur battery reaches a level that is equal to or higher than a predetermined voltage or that is equal to or lower than a predetermined current. Thus, an occurrence of a malfunction of a single cell in the sodium sulfur battery is automatically detected by monitoring a discharging current during energy discharge from the sodium sulfur battery to the heater.